The present invention relates to a seat belt retractor, and particularly, to a motorized seat belt retractor, which can change its operating condition to correspond to the state of an occupant wearing a seat belt or to external signals.
In a seat belt retractor provided in an automobile, it is preferable to retract an excess amount of a seat belt after the seat belt is pulled and a tongue is engaged with a buckle device, without applying too much stress to the chest or other portions of an occupant normally wearing the seat belt.
Normally used for winding up a seat belt in a seat belt retractor is a biasing force of a single return spring. When a spring providing a small biasing force is employed in order to reduce stress applied to the chest of an occupant, the force for winding or retracting the seat belt becomes weak, thus lowering its operation and its accommodation to the retracted state.
To the contrary, when a spring for providing a large biasing force is employed in order to provide an enough winding force for its retraction, the stress to be applied to the chest of the occupant normally wearing the seat belt is increased.
Since a single return spring is employed, a conventional seat belt retractor has a problem that the biasing force of the return spring increases as the amount of withdrawal of the webbing (hereinafter, the term xe2x80x9cwebbingxe2x80x9d is used for referring to the belt itself in this specification) is increased because the return spring is repeatedly wound.
As one of means for solving the problem with regard to the winding of a webbing caused by using a single return spring, the applicant has developed a tension reducing mechanism for reducing the retraction force, which comprises two rope pulleys of truncated cone shape with a helical guide groove (see Japanese Patent No. 2711428).
On the other hand, the applicant also has developed a seat belt retractor in which a tension control for a seat belt is performed by a built-in electric motor to improve comfortableness as one of functions when an occupant normally wears, and to correspond to a distance from another vehicle running in front of or behind its own vehicle (see Japanese Unexamined Patent Publication No. H09-132113).
In the seat belt retractor disclosed in the aforementioned publication No. H09-132113, an ultrasonic motor is employed as a driving unit for the tension control and a spring mechanism is provided as a main winding mechanism. The motor is employed for controlling the tension on the seat belt when the occupant wears the seat belt, and for controlling the winding and unwinding of the seat belt to correspond to external signals inputted during running of the vehicle. Therefore, it is required to rotate the motor in reverse. This makes the control circuit Complex. Since the operation of winding up the seat belt is performed by two circuits, there is also a problem that it is hard to smoothly switch the operation of winding up the seat belt according to the inputted external signals.
When the external signals from sensors mounted on the vehicle are set to function as the trigger of the motor, a speed-reduction gear to be driven by the motor employed in the seat belt retractor can not cope with a wide range of a winding mode because the reduction range allowed by the speed-reduction gear is limited.
Therefore, it is an object of the present invention to solve the problems of the prior art and to provide a seat belt retractor in which a seat belt can be wound by only a motor with a simple control circuit and a driving mechanism, and the winding mode can be set widely to correspond to external signals inputted when an occupant wears the seat belt and a vehicle runs, and to provide a control method for the seat belt retractor.
Further objects and advantages of the invention will be apparent from the following description of the invention.
To solve the aforementioned problems, the present invention provides a seat belt retractor comprising: a base frame; a spool journalled at a spool shaft to the base frame; a speed-reduction mechanism; and a motor for rotating the spool via the speed-reduction mechanism for winding up a webbing onto the outer periphery of the spool. The speed-reduction mechanism has two transmission trains with different reduction gear ratios, switching means for switching the transmission trains to one of the trains corresponding to a driving signal obtained from external units, and resistant torque means having a preset value so that a transmission element common to the two trains transmits rotation of the motor when the torque of the rotation is smaller than the preset value.
The driving signal is preferably set according to a state signal obtained by movement of an occupant wearing the seat belt and an external signal obtained from detecting means installed in a vehicle during the running of the vehicle.
It is preferable that when the state signal is received, the rotational torque is reduced at a small reduction gear ratio to be smaller than the preset value of the resistant torque means, and the spool shaft is rotated with the reduced rotational torque for winding up the webbing.
It is preferable that when the external signal is received, the switching means switches the transmission trains so that the rotational torque is increased at a large reduction gear ratio to exceed the preset value of the resistant torque means to wind the webbing.
It is preferable in the above that the resistant torque means is a slip mechanism comprising a viscosity resistant element disposed on the trains of the speed-reduction mechanism.
It is also preferable that the switching between the trains is achieved by actuating the switching means according to the driving signal when the external signal is received and securing a part of the transmission element disposed in the speed-reduction mechanism.
It is preferable that the switching means comprises a pawl mechanism which engages one of ratchet teeth formed on the outer periphery of an internal gear in a planetary gear unit to stop the rotation of the internal gear.
It is also preferable that the switching means comprises a pawl mechanism having two pawls journalled by shafts, which cooperate to engage ratchet teeth of the internal gear to stop rotation of the internal gear.
In this case, it is preferable that the two pawls are a first pawl and a second pawl, and the first pawl pivots about the shaft by excitation of a solenoid to engage one of the ratchet teeth and, after that, the first pawl makes the second pawl to engage another of the ratchet teeth.
It is preferable that a spring connected to the first pawl biases the first pawl after cancelling the excitation of the solenoid with the result that the second pawl disengages from the ratchet teeth.
It is preferable that the rotation of the spool is reduced through a reduction gear train, and the spool shaft is provided with a spool rotation detecting unit for detecting a rotational direction and a stopping state of the spool.
It is preferable that the withdrawal of the webbing is detected by the start of rotation of the spool and acts as a trigger for turning ON a power source of a control circuit for driving the motor.
The present invention also provides, as another invention, a seat belt retractor comprising a base frame, a spool journalled at a spool shaft to the base frame, around which a webbing is wound, a return spring for winding up the webbing by its return force, and a motor with a speed-reduction mechanism for switching the winding of the webbing, wherein the speed-reduction mechanism has a transmission train with a predetermined reduction gear ratio and switching means for connecting the transmission train to the spool shaft corresponding to a driving signal obtained from an external unit.
The driving signal is preferably set according to a state signal obtained by movement of an occupant wearing the seat belt and/or an external signal obtained from detecting means installed in the vehicle during running of the vehicle.
It is preferable that when the external signal is received, the switching means switches the transmission trains so that the webbing is wound by the return spring at a large reduction ratio.
It is preferable that the switching of the transmission train is achieved by actuating the switching means according to the driving signal when the external signal is received, and securing a part of transmission elements disposed in the speed-reduction mechanism.
It is preferable that the switching means comprises a pawl mechanism which engages one of ratchet teeth formed on an outer periphery of an internal gear in a planetary gear unit to stop the rotation of the internal gear.
The present invention also provides, as a further invention, a seat belt retractor comprising: a base frame; a spool journalled at a spool shaft to the base frame around which a webbing is wound: a return spring for winding up the webbing by its return force: and a motor with a speed-reduction mechanism for winding up the webbing at a low speed for fitting the webbing to an occupant, wherein the speed-reduction mechanism has a contrate gear with a predetermined reduction gear ratio and wherein the webbing, is wound by driving the motor via the contrate gear.
The present invention also provides, as an invention for efficiently operating the above retractor, a control method of a seat belt retractor comprising: receiving an external signal outputted to correspond to a state signal according to the movement of an occupant wearing a seat belt and/or an external signal from detecting means installed in a vehicle during running of the vehicle; switching transmission of a rotational torque of a motor to a predetermined reduction ratio corresponding to the state signal or the external signal, or controlling the driving state of a motor to change its torque to a predetermined value to control the rotation of a spool; and winding the webbing onto the spool.
It is preferable that based on a state signal obtained by detecting withdrawal of the webbing while the webbing is in the wound state, the rotation of the motor is stopped.
It is preferable that based on a state signal obtained by detecting that a tongue is engaged with a buckle, the motor is driven with such torque as to wind the webbing to fit the webbing to the occupant""s body.
It is preferable that based on a state signal obtained by detecting that the fitting of the webbing is accomplished, the torque of the motor is reduced or the drive of the motor is stopped.
It is preferable that based on a state signal obtained by detecting that the withdrawal of the webbing is cancelled, the motor is restarted to wind the webbing.
It is preferable that when the withdrawal of the webbing is made and stopped while a tongue is engaged with a buckle, the winding of the webbing is started to fit the webbing to the occupant and, after that, the torque of the motor is reduced or the drive of the motor is stopped.
It is preferable that the torque of the motor for fitting the webbing to the occupant is set lower than the torque of the motor for fitting the webbing to the occupant just after the tongue is engaged with the buckle.
It is preferable that based on a state signal obtained by detecting that a tongue is disengaged from a buckle, the motor is activated to wind the webbing into the retractor.
It is preferable that at the same time of or after a predetermined period of time from the detection of the withdrawal of the webbing, a power source of a control circuit for operating the motor is turned ON.
It is preferable that at the same time of detection that the winding of the webbing is accomplished, or detection that the webbing has not been withdrawn for a predetermined period of time, a power source of the control circuit for operating the motor is turned OFF.
It is preferable that the control method further comprises: switching the torque of the motor to a preset value or a variable value by switching means according to an obtained external signal to wind the webbing by the motor.
It is preferable that the control method further comprises: providing a mode in which the switching means is not returned so as to hold the driving state of the motor after being switched.
It is preferable that the control method further comprises: providing a mode in which the switching means is not returned so as to hold the driving state of the motor after being switched through torque resistor means of the speed-reduction mechanism.
It is preferable that the mode is a holding mode during running of the vehicle or a child seat fixing mode.
It is also preferable that the control method further comprises: transmitting a command signal of the motor corresponding to the external signal as information to be used for one or all of the seat belt retractors installed in vehicle seats in a communicating method capable of recognizing the seats so as to perform the mode setting for the vehicle seats.